An alkyl vinyl ether/maleic anhydride copolymer is a copolymer-anhydride having a linear molecular structure and a water-soluble high molecular weight electrolyte copolymer compound which has been used as, for example, surface active compounds, a component of glass cleaners, a component of additives to builders for a detergent, and as sequestering agents themselves.
In particular, methyl vinyl ether/maleic anhydride copolymer has recently been used as a component of water-soluble additives to agents to be coated on a sticking plaster (e.g., under the name "Pappu-zai" in Japan) by mixing with endermic liniments and glues because of the copolymer having an excellent non-toxic property with respect to the human body, an excellent chemical stability for a long period of time, an excellent stickiness, an aggregating ability, a water absorbing ability, restrippability, etc.
Furthermore, copolymers in which maleic anhydride units are partially or completely ring-opened with water, alkalis, or various alcohols, are also widely applied as thickening additives for water soluble materials, components of additives to adhesives, an anti-sticking additives to detergents, and components of additives to spray-type fixing agents for human hairs.
Hitherto, it is well known that an alkyl vinyl ether/maleic anhydride copolymer has been produced by free radical copolymerization of an alkyl vinyl ether monomer with maleic anhydride monomer in the presence of a free radical catalyst and in the absence of molecular oxygen at temperatures of from 40.degree. to 120 .degree. C. in organic solvents.
Sedimentation polymerization in which the reaction is carried out in the presence of a solvent capable of dissolving maleic anhydride which is a starting material, and incapable of dissolving an alkyl vinyl ether/maleic anhydride copolymer is also known.
Furthermore, a process such as the solution polymerization in which the reaction is carried out in the presence of a solvent capable of dissolving both of maleic anhydride which is a starting material and an alkyl vinyl ether/maleic anhydride copolymer is known.
For example, the sedimentation polymerization in benzene which is an aromatic hydrocarbon, is disclosed in U.S. Pat. Nos. 2,782,182, 3,030,343, 3,499,876, 3,532,771, 3,553,183 and 4,900,809, DE 3302495 and Japanese Examined Patent Publication (Kokoku) No. 29193/1970 and Japanese Unexamined Patent Publication (Kokai) Nos. 204910, 204911 and 204912 / 1989, etc.
U.S. Pat. Nos. 3,499,876, 3,532,771, 3,553,183, DE 3302495 and Japanese Unexamined Patent Publication (Kokai) Nos. 204910, 204911 and 204912 / 1989 disclose also the use of toluene and xylene which are aromatic hydrocarbons.
Furthermore, U.S. Pat. No. 3,532,771 also discloses the use of hexane, heptane and nonane which are an aliphatic hydrocarbons.
Still further, U.S. Pat. No. 3,499,876, DE 3302495 and GB 1117515 disclose the use of dichloroethane which is an halogenated aliphatic hydrocarbon.
DE 3302495 discloses the use of trichloroethane or carbon tetrachloride which is an halogenated aliphatic hydrocarbon.
In addition, there are disclosed the use of an admixture composed of 1,1,2-trichloro-1,2,2-trifluoroethane and dichloromethane in Japanese Unexamined Patent PublicatiOn (Kokai) No. 145714/1980; the use of an ester compound having the carbon number of 51 in DE 3736996; the use of methyl acetate, ethyl acetate, isopropyl acetate and or methyl propionate in Japanese Unexamined Patent Publication (Kokai) Nos. 204910-204912/1988; the use of an allyl vinyl ether in GB 906230, DE 3712265 and DE 3108100; and the use of a sterically hindered monoether in WO 9012817.
On the other hand, examples of the solution polymerization using methyl acetate or ethyl acetate are disclosed in U.S. Pat. NO. 4948848, using acetone in U.S. Pat. Nos. 3499876 and 4948848, and using methyl ethyl ketone in U.S. Pat. No. 3499876.
However, when conventional solvents which are harmful to the human body such as benzene, toluene, xylene, carbon tetrachloride, dichloroethane, trichloroethane, etc., are used in the reaction, it has been difficult to economically reduce the residual solvent in the product to zero, even though being-carefully dried in the drying step after the completion of the reaction.
As described hereinabove, an alkyl vinyl ether/maleic anhydride copolymer is often used as a component in a detergent which directly contacts with the human body (particularly, skin).
However, the above reaction methods using harmless solvents also have the disadvantages as described below.
For example, in the case of the reaction using an ester compound such as methyl formate, isopropyl acetate, butyl acetate and or methyl propionate, etc., the resulting copolymer readily adheres to the inside of the reaction vessel during reacting, resulting in a problem of handling.
In the case of the reaction by the solution polymerization using methyl acetate, ethyl acetate, acetone and or methyl ethyl ketone, etc., a considerably large amount of energy is required on removing the solvent as compared with sedimentation polymerization. Furthermore, in the case of the method using an expensive solvent such as an admixture composed of 1,1,2-trichloro-l,2,2-trifluoroethane and dichloromethane, a sterically hindered monoether compound or an allyl vinyl ether, etc., there is a problem from the viewpoint of cost, because of a certain extent of the unavoidable loss on removing the solvents.
Still further, the conventional methods, particularly in the case of carrying out the reaction using benzene, can only provide copolymer having a lower bulk density, which results in inefficient shipping.
Accordingly, there is a need for the development of a method in which a copolymer having a higher bulk density can be prepared.
In view of these objections, and as a result of extensive investigations, the present inventors have now completed the present invention.